Eavesdropping using microwaves - addendum

Microwave eavesdropping has a long and clandestine history. And there’s an odd connection between music and microwave eavesdropping. Mention the name Leon Theremin and many people will immediately think of the musical instrument bearing his name. Born Lev Sergeyevich Termen, his contributions to the former Soviet Union spanned the history of the Soviet Union from the original Bolshevik Revolution to the collapse of the USSR.

Termen was a mathematics prodigy who was drawn into the emerging field of electronics. Termen came across the basic principle for one of the first electronic musical instruments while working on television. The theremin (Termen called it an "etherophone") produced music with modulated static. Players used two antennae, one for pitch and one for volume, producing musical tones by waving their hands around the antennae. Skilled musicians reportedly could recreate the sounds of many existing instruments.

Termen traveled to the US under the pretense of demonstrating the wonders of the theremin. The real purpose of his trip was to collect information about German and US technology and manufacturing, reporting them to the USSR. His return to the Soviet Union came at an inopportune time in history. Because Termen had spent years in America, he became a suspect of counterrevolutionary thoughts and was sent to the prison camp/gold mine of Kolyma. Most non-Russians believed that Termen had disappeared in the middle of the night, kidnapped by Soviet agents.

Ultimately the pogrom against anti-Stalinists left so few experts available for espionage work that Termen was pressed into service. Among his inventions was a passive cavity audio bug.

While there are a great many myths about when and where the bug was most famously deployed, the NSA provides a definitive history.
From the NSA web site:

“On August 4, 1945, Soviet school children gave a carving of the Great Seal of the United States to U.S. Ambassador Averell Harriman. It hung in the ambassador's Moscow residential office until 1952 when the State Department discovered that it was 'bugged.'

The microphone hidden inside was passive and only activated when the Soviets wanted it to be. They shot radio waves from a van parked outside into the ambassador's office and could then detect the changes of the microphone's diaphragm inside the resonant cavity. When Soviets turned off the radio waves it was virtually impossible to detect the hidden 'bug.' The Soviets were able to eavesdrop on the U.S. ambassador's conversations for six years.”

Figure 1 Carved Great Seal of the United States hid resonant cavity bug.

Figure 2 Hiding place for resonant cavity bug.

The existence of the bug was accidentally discovered by a British Radio Operator when he heard conversations on an open radio channel.

In 'Spycatcher' Peter Wright, former assistant director of MI5, detailed how he uncovered the workings of the microwave bug. He had been working with resonance cavities trying to create practical spy equipment. The existence of the Russian device proved the concept possible, but he was stymied by how the device worked. Nothing that he did to it caused it to broadcast any audio. It was about eight inches long, with an antenna on top which fed into a cavity. Inside the cavity was a metal mushroom with a flat top which could be adjusted to give a variable capacity. Behind the mushroom was a thin diaphragm, to receive speech. They had assumed that the metal plate needed to be opened out to increase resonance, but in actual operation the closer the plate was to the mushroom the great the sensitivity. Inspiration caused Wright to try a new experiment that activated the bug.

The resonant cavity transmitter is simple technical device called a passive radiator. A layer of thin metalized material is stretched across a closed metal tube. The size of the tube determined its resonant frequency. An antenna, is attached to the base of the cavity. The cavity is irradiated with a beam of radio frequency energy from an external source. The size of the cavity and the length of its antenna are designed so that a harmonic of the inbound radio frequency energy is rebroadcast. The metalized diaphragm acts as a transducer, and the audio range energy modulates the returned radio frequency signal that, in turn, is picked up by a receiver in a nearby listening post. It is important to note that the microwave signal that “powers up” the device is not the same frequency as the outbound signal.

Figure 3 Resonant cavity bug structure.

Figure 4 How the bug worked.

The microwave eavesdropping equipment discussed in “Eavesdropping using microwaves” uses no resonant cavities. It is very useful because there is no need to plant a device in the room. US Patent 4359683 filed in 1980 gives a preview of the new invention. In the patent, the inventor John W.H. Chivers of Rolls Royce, details a microwave interferometer for measuring distances, but fails to disclose any idea that the equipment might be useful for intercepting sounds. In US Patent 4280055 inventor Reinhold Gerharz takes a different view of microwaves to describe an imaging device. Neither of these two patents tell us how to use microwaves to directly eavesdrop on sounds without a resonant cavity.

A commercial product gets us closer to the eavesdropping solution. 2010/2010A - Doppler Stethoscopes are used to detect motion. The 2010 is a stand-alone probe designed for use with a high gain amplifier. The units generate a small amount of radio energy which penetrates any soft sided package such as an attache case, suitcase, wooden or cardboard box, and returns a signal which indicates movement within the package. This includes many types of watch mechanisms, mechanical timers, radio control escapements and tape recorder motors. Still, the stethoscope does not directly extract speech.

In US Patent Application 2005/0220310 William R. McGrath details how microwaves can be reflected to intercept sounds. The important part of this patent is the detailing of how microwaves can be reflected by an object and signal processed to recover audio information.

The continuing development of audio intercept technology has important legitimate uses. But the more technically sophisticated approaches that can be built simply carry with them an increased risk of abuse.

I don't know. Try putting an rf signal of a low frequency on one of the strings of the harp using a small alligator clip connected to a wire and oscilator. Voices in the room should cause the harp string to vibrate which would then modulate the rf wave. With a radio receiver you could then listen in.

The following is from a usenet post from 1987:
In article <2...@phri.UUCP>, r...@phri.UUCP (Roy Smith) writes:
> > Wasn't the U.S. Embassy in Moscow being snooped on a couple of years
> > ago with *microwaves* being bounced off windows?
It probably still is. The microwave energy is probably being used
for at least three purposes:
1. To excite passive tuned-cavity listening devices. A sharply tuned
cavity is fitted with a metallic microphone diaphragm. Microwave energy is
directed toward the cavity using a directional antenna from the monitoring
location. The microwave oscillator is connected to the antenna using a
directional coupler having forward and reverse power detectors. The
instantaneous VSWR is an indication of diaphragm displacement as the cavity
power absorbtion changes through detuning. Modulation of the microwave
oscillator, with appropriate detection, can also be used to improve S/N ratio
of the detected signal.
2. To excite listening devices which use received energy to power a
transmitter on a different frequency, which is then detected by a receiver
at the monitoring location. This technique may be implemented by a tuned
cavity that is fitted with a varactor diode leading to a second tuned
cavity at a higher frequency. The second tuned cavity is fitted with a
metallic microphone diaphram. The "retransmitted" frequency is often a
simple second or third harmonic of the excitation frequency; depending upon
the type of diode and what frequency at which it is designed to oscillate,
the retransmitted frequency can also be a small frequency shift of the
excitation signal. This principle has been used for a number of years as
a passive military radar beacon (sans microphone :-).
3. To measure the micro-displacement of some reflecting metal object
in the subject room; this displacement is presumably the result of sound
vibration within the room. The reflected signal is modulated both by
sound amplitude and sound frequency; the reflected signal is detected as
phase modulation and corresponds to the instantaneous velocity of the
reflecting surface. This technique has the advantage of no advance
"planting" of a listening device in the room. The principle behind this
technique has been used for industrial vibration measurement for at
least 40 years.
> The story I heard (about as reliable as any Nth-hand info) was that
> the the Russians presented the American embassy folks with some sort of
> carved wood plaque of an american bald eagle, or something like that, as a
> gift. Of course, the embassy folks gave it to the electronics types to
> look at to make sure it wasn't bugged and then hung it up in the office.
> It never occured to them that the sheet metal plate on the back of it might
> actually be a microwave reflector... Memory is hazy, but I seem to
> remember hearing this 5-7 years ago, and the story was at least a few years
> old by then. Believe it at your own risk.
It's a true story, and it happened during the 1950's at the United
Nations; most people in the know call this "The Great U.N. Seal Caper". :-)
It seems that the Russians gave the Secretary General of the U.N.
a nice wooden plaque which contained a passive-cavity listening device
whose principle I described in (1) above. It operated undetected for at
least two years. The frequency of operation was around 330 MHz (which is
not truly microwave, but the principle is still the same). The Russians had
a high-power transmitter concealed in a truck that they would park in close
proximity to the the U.N. building when they wanted to monitor the device.
I don't know how much transmitting power was used, but considering the lack
of directivity at UHF frequencies I would guess it to be several hundred
watts CW. I seem to recall that the device was discovered only because the
truck was discovered, thereby triggering a thorough search of the building.
You have to admit, them Rooskies have guts. :-)
As can be seen, your article is fairly well written but poorly researched. In addition, the Magrath patent has more to do with the politics of ignorance and potential financial gain more than anything else. Your article mentioned a commercial product called the Doppler Stethoscope. In regards to this device it was stated that it will not directly extract speech. I have first hand knowledge of this device and I can state without any doubt that it will in fact recover speech. Not from any great distance, but yes, it will do this. There are several different copies of this device that an electronics individual has been attempting on a website....try this out: http://projects2.gbppr.org/mil/index.html Look at the various links on the page especially at the bottom of the page. There are some recordings of the device operating with various levels of sucess. Basically, the priciple involves a device called an interferometer, which is just a fancy term for a beam splitter. To make one of these all you really need to do is take an rf signal and split it into two legs of different lengths with one beam being the reference and the other beam being the reflection and then recombine them and extract and demodulate the phase shift to recover the amplitude which contains vibrations caused by speech. You can do it with a couple of FRS radios and some odds & ends. The Led & Laser systems that do this, which were also invented by the Soviets by the way, use the same phase/demodulation scheme as well. There are also ultrasonic flood versions of these devices too, but I do not think they ever got much use. Hope this helps.

Does anyone know how (or whether) an orchestral harp could be bugged? Or can you think of an instrument suitable for bugging--one not likely to be kept in a case most of the time (like a guitar), which might interfere with the task at hand?
I'm writing a screenplay in which someone will bug another person's instrument (not while it's being played), but while it's in their apartment or car. Thanks to anyone who can help. Most photos I've found of orchestral harps don't show the sound holes I was looking for, through which a bug could be planted inside.